1. Field of the Invention
This invention relates to the removal of sulfur oxides from a flue gas stream by injection of a dry soda ash sorbent into the hot gas stream. The absorptive soda ash sorbents used in this invention are porous calcines derived from NaHCO.sub.3 -containing compounds.
2. Description of the Prior Art
Dry sorbent injection is receiving increased attention as a method for removing sulfur oxides from the stack gases of coal-fired utility and industrial plants, especially those using low sulfur coals. Flue gas desulfurization (FGD) systems in commercial use are generally based on wet scrubbing or spray drying processes. Evaluations of dry sorbent injection, however, have revealed its advantages of operational simplicity and reliability, attractive equipment and labor costs, and lower water consumption, over the present conventional FGD systems.
In dry sorbent injection, a suitable sorbent is injected as a dry powder into a SO.sub.2 -containing flue gas stream and is then collected, along with fly ash in the hot flue gas stream, in a fabric filter baghouse.
Sulfur dioxide in the hot flue gas stream reacts with the injected sorbent, while the latter is fluidized in the ductwork and also when the flue gas passes through the baghouse-collected filter cake. The baghouse filter cake, a mixture of fly ash and sorbent-SO.sub.2 reaction product, is periodically removed for disposal.
Sodium compounds are currently favored as dry injection sorbent candidates, with nahcolite (crude sodium bicarbonate) being described as more effective in SO.sub.2 -absorption than trona (crude sodium sesquicarbonate), but with soda ash (commercial sodium carbonate) being characterized as essentially ineffective for removing SO.sub.2, as noted by M. E. Kelly et al. in their "Third Survey of Dry SO.sub.2 Control Systems," EPA-600/7-81-097 (June 1981), and confirmed in a full-scale demonstration reported by L. J. Muzio et al. in "Dry SO.sub.2 Particulate Removal for Coal-Fired Boilers, Vol. 2: 22-MW Demonstration Using Nahcolite, Trona, and Soda Ash," Electric Power Research Institute Report CS-2894, Volume 2, June 1984.
The parameters important in dry injection FGD procedures are still being investigated, and consequently the issued patents directed to this FGD technique are few in number and contain broad, generalized disclosures. U.S. Pat. No. 4,018,868 issued to Knight describes a dry injection process in which crushed nahcolite is utilized for removing sulfur dioxide from gas streams.
Other patent references contain disclosures suggesting the use of calcined nahcolite or of calcined trona for flue gas desulfurization because of the increased porosity that can result from decomposition of the bicarbonate or carbonate. These patents, like U.S. Pat. No. 3,821,353 issued to Weichman and U.S. Pat. Nos. 4,385,039 and 4,481,171 issued to Lowell et al., provide little specific information about the porosity characteristics of the calcines or about the effect of porosity-related parameters on the desulfurization performance of the calcined materials.
Still other patents which concern calcination of NaHCO.sub.3 -containing compounds like trona, typically for use in the "monohydrate" soda ash process, do not suggest that such calcines have any utility for flue gas desulfurization. U.S. Pat. No. 3,869,538 issued to Sproul et al. describes the use of a fluid bed calciner, in lieu of conventional rotary calciner, for processing trona in the monohydrate soda ash process.
Likewise, U.S. Pat. Nos. 3,333,918 and 3,482,934 issued to Gancy and DiBello describe the preparation of an "active sodium carbonate" intermediate from sodium bicarbonate, sodium sesquicarbonate, or sodium carbonate monohydrate, that is converted to a dense soda ash product; but there is no suggestion that either the dense soda ash or the intermediate would have utility in flue gas desulfurization.
Studies reported in the FGD literature indicate that calcination of NaHCO.sub.3 -containing compounds, to prepare FGD dry sorbents, is desirably avoided since the resultant calcines are not efficient FGD sorbents. In one evaluation of calcined nahcolite as a dry injection FGD agent, it was determined that the efficacy of calcined nahcolite was significantly less than that of uncalcined nahcolite or trona; see L. J. Muzio et al., "Bench-Scale Study of the Dry Removal of SO.sub.2 with Nahcolite and Trona," Electric Power Research Institute Report CS-1744, March 1981.
In an effort to explain the poor performance of calcined sodium compounds, investigators researching the use of soda-type dry sorbents in dry injection FGD processes have noted that absorption of SO.sub.2 from the gas stream (occurring both while the sorbent is fluidized in the gas stream and while deposited in the collected bag filter cake) appears to result in the progressive decline in the sorbent's SO.sub.2 removal efficiency. L. J. Muzio, et al. in their previously-mentioned EPRI Report CS-2894 (in Section 5) speculate that pore plugging is the factor limiting the complete utilization of sodium in the dry sorbent, due to formulation of a barrier layer of reaction product, Na.sub.2 SO.sub.4, through which SO.sub.2 must diffuse in order to react with unreacted sodium (carbonate) in the sorbent.
These researchers accordingly believe that the best sorbents are uncalcined bicarbonate-containing compounds like nahcolite and trona. When used as sorbents in dry injection FGD, such uncalcined materials avoid premature pore plugging because of the bicarbonate decomposition reaction, evolving water and carbon dioxide, that occurs concurrently with the desulfurization reaction.
The present invention concerns a flue gas desulfurization method that utilizes calcined compounds like calcined trona and calcined nahcolite as sorbents in a dry injection process which, contrary to the adverse disclosures of the prior art, nevertheless provides highly efficient removal of sulfur oxides from flue gas streams.